* S.Africa mining firms/unions reach 'in principle' undertakings
* U.S. retail sales, jobless claims below expectations (Releads, updates prices, adds comment)
By Clara Denina
LONDON, June 12 (Reuters) - Platinum and palladium posted their biggest daily losses for nearly a year on Thursday, after South African producers said they had reached "in principle" undertakings with the union to end a crippling five-month strike.
The Association of Mineworkers and Construction Union (AMCU) took the wage offer from Anglo American Platinum Ltd, Impala Platinum Holdings and Lonmin Plc to their 70,000 striking members. That fuelled expectations that an end to the strike that has disrupted global production could be in sight.
Spot platinum fell 3.3 percent to a one-week low of $1,427.50 an ounce, posting its biggest daily loss since June 2013. It was trading down 2.2 percent at $1.443.40 by 1325 GMT.
Spot palladium fell as much as 4.6 percent to $817.60 an ounce, its lowest since May 20 in earlier trade. It was down 2.6 percent at $835.05 at 1325 GMT.
On Wednesday, palladium had risen to $862.50 an ounce, its highest level since February 2001 after a fresh round of wage talks broke down.
"The drop will continue if there are more encouraging news on the end of the strike, especially for palladium, which has risen more than platinum," Natixis analyst Bernard Dahdah said.
"... Our calculations show that producers' stocks are available until end-August and that gives time to restart operations and keep the market supplied."
Mitsubishi Corp analyst Jonathan Butler estimated that 600,000 ounces of palladium have been lost since the strikes started in January and 1.1 million ounces of platinum.
Palladium's fundamentals, however, remain strong on buoyant industrial demand from auto makers and investment buying through exchange-traded funds (ETFs), analysts said.
The metal, mainly used in gasoline-powered catalytic converters to clean exhaust emissions, has gained 15 percent this year, overshadowing gains of its sister metal platinum, which has risen about 5 percent.
"Around 800,000 ounces of palladium have been accumulated in the two new palladium ETFs pretty much after 2-1/2 months," Butler said.
Standard Bank's Johannesburg-listed AfricaPalladium ETF and Absa Capital's New Palladium were launched in March to meet demand from South African investors seeking a domestic palladium fund.
"The autocatalyst market is very much dependent on gasoline-vehicle sales, which are really soaring in the main two markets China and the United States," Butler added.
GOLD GAINS AFTER U.S. DATA
Gold extended earlier gains after both U.S. retail sales and jobless claims came in below expectations, weighing on the dollar.
Retail sales rose 0.3 percent in May, half of the growth rate that had been expected, while the number of Americans filing new claims for unemployment benefits unexpectedly rose last week.
Spot gold added 0.5 percent an ounce to $1,266.10, while U.S. gold futures for August delivery rose by the same margin to $1,266.30 an ounce.
The dollar turned 0.1 percent weaker against a basket of currencies after the data. A weaker dollar makes gold cheaper for holders of other currencies.
Silver rose 0.6 percent to $19.26 an ounce. (Additional reporting by Lewa Pardomuan in; Singapore; Editing by Susan Thomas and Susan Fenton)